The present invention relates to a power factor corrector and, more particularly, to a circuit for correcting the power factor of an alternating current signal to be applied to an AC load, most typically a capacitive load.
The term "power factor" is used to distinguish between the real and apparent power drawn by a load. Where a voltage is applied across a resistive load the current varies in accordance with the voltage and the real power is the same as the apparent power. However, where, for example, voltage is applied across a capacitive load, such as capacitive input filter of power supplies, the current does not vary in the same manner. Rather, the current flow may be characterized by a sharp pulse during each portion of the voltage half cycle, resulting in high peak currents instead of the current being distributed more evenly over the period of the input voltage waveform. The disadvantages of such high peak currents is well known to those of ordinary skill in the art and will not be described in detail at this point. In general, such high peak currents result in conditions of high peak power requirements disproportionate to the total power delivered. Such high peak power requirements impose undesirable requirements upon the power supply company and may even cause dangerous conditions within the power supply circuit.
A number of power factor correction circuits have been previously proposed and are in current use. One of the most common methods of improving the power factor of the circuit is to incorporate inductive filters into the input circuit. In some cases contemporary circuits utilize tuned filters or resonant energy storage systems, which use passive components such as inductors and capacitors to trap the higher order harmonics to reshape the input current waveform to approximate that of the a pure sine wave. Such circuits advantageously do not require active components. However, they typically require large and heavy components and/or are characterized by additional losses associated with circulating current in resonant circuits. Moreover, such circuits are typically frequency sensitive and are limited in their tolerance of inductive loads, such as fans or blowers.
Another contemporary technique for power factor correction uses active circuitry to convert the AC input to a DC voltage, which may be outputed as a regulated DC power supply or may be modulated in accordance with the AC voltage waveform. Such circuits are commonly current controlled and programmed to draw current from the AC line in almost pure sine wave. Circuits of this type find common application as an integral portion of a regulated DC power supply. Some of the disadvantages associated with such circuits includes lower reliability, due to the use of active elements and the requirements that the active elements handle all of the throughput power of the system. In addition, the high frequency switching utilized in such circuits may produce EMI emissions that require additional filtering. Losses associated with such systems and the requirement to utilize expensive high frequency switching components further limit the suitability of circuits of this type for many applications.
Accordingly, there is a need for a reliable power factor correction system which utilizes a minimum of active circuitry to reduce input current distortion and is useful to accommodate different input signals. Preferably such a system should produced an AC output that may be used to drive a variety of inductive and capacitive loads, and is tolerant of frequency deviations in the input signal. The present invention is directed to a circuit and method of power factor correction which satisfies these and other requirements in an efficient and reliable system.